Excessive extracellular matrix and serum proteins accumulate in the glomerular mesangium in diverse glomerular disease processes. Extracellular matrix (ECM) turnover is regulated by a balance of synthesis and degradation. ECM composition, cell shape and cell adhesion are interdependent expressions of cell function regulated by autocrine and paracrine mechanisms. The importance of intraglomerular coagulation and fibrin deposition in glomerular disease has been the subject of many reviews. Thrombin has multiple hormone-like effects on cells including contraction, proliferation, chemotactic and growth factor secretion and regulation of ECM synthesis. Recently a thrombin receptor has been cloned. Thrombin cleaves the receptor near its amino terminus exposing a new terminal peptide (tethered ligand) that binds the active site. A 14 amino acid synthetic peptide corresponding to the tethered ligand is a potent agonist of the thrombin. Previously, it was not possible to clearly separate receptor-mediated and proteolytic effects of thrombin. The broad long term objective of this proposal is to delineate the mechanisms (receptor-mediated and proteolytic) by which thrombin regulates ECM composition. Specifically, the regulation of ECM synthesis and degradation in cultured human glomerular mesangial cells will be addressed by investigating the following hypotheses: 1) Thrombin regulates the synthesis and degradation of extracellular matrix protein through receptor-mediated and proteolytic mechanisms. The abilities of delta-thrombin and thrombin receptor agonists to alter the synthesis, degradation and net accumulation of ECM proteins and plasminogen activators and their inhibitors by human mesangial cells will be compared. 2) Thrombin receptor is regulated through synthesis, internalization and degradation. The mechanism of activation of the thrombin receptor is novel, involving cleavage by thrombin near its amino terminus exposing an attached peptide capable binding to the receptor active site ("tethered ligand"). This mechanism has important implications for regulation of receptor activity, since many receptors can be activated by a single molecule of thrombin, the tethered ligand formed cannot dissipate through diffusion, and repeated stimulation must require synthesis of new receptor. Therefore, antibodies will be developed and receptor synthesis, degradation, distribution and function will be studied. These studies will aid in our understanding of potentially important mechanisms involving thrombin that may underlie regulation of mesangial cell ECM metabolism in glomerular sclerotic disease processes.